System and method of shaping a power spectrum in pwm amplifiers

ABSTRACT

In a particular embodiment, a circuit device is disclosed that includes a data generator adapted to output a random pulse sequence having a particular spectral shape. The circuit device further includes a pulse edge control circuit to selectively apply a carrier suppression operation to at least one pulse-width modulated (PWM) signal in response to the random pulse sequence to produce at least one modulated PWM output signal. The spectral energy associated with a PWM carrier of the modulated PWM output signal at a carrier frequency and associated harmonics is changed such that the modulated PWM output signal has a spectral shape defined by the particular spectral shape.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a non-provisional patent application of andclaims priority from U.S. provisional patent application Ser. No.61/072,563, filed Apr. 1, 2008 and entitled “COMMON MODE CARRIERSUPPRESSION AND SPECTRAL SHAPING IN CLASS D AMPLIFIERS,” the content ofwhich is hereby incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

This disclosure generally relates to a system and method of shaping acommon mode spectrum in pulse-width modulated (PWM) amplifiers.

BACKGROUND OF THE DISCLOSURE

Conventionally, class D audio amplifiers have the benefit of high powerefficiency, but such amplifiers can also have a drawback in terms ofelectromagnetic interference (EMI), which can interfere with nearbywireless receivers, violate Federal Communication Commission (FCC)emission limits, or any combination thereof. Audio Class D amplifiersoften switch at a frame rate of a few hundred kHz, and common modeenergy at a carrier frequency and its harmonics can fall directly in theamplitude modulated (AM) radio frequency band, interfering with nearbyAM receivers.

FIG. 1 illustrates a graph 100 of a “BD modulation” employed by manyclass D amplifiers. Class BD-D modulation varies pulse widths of twopulse waves that are time-aligned and often nominally centered within apulse-width modulated (PWM) frame, which has a frame width (T). Forpositive input signals, the pulse width PWM B signal 102 that drives thehigh side of the bridged output (typically referred to as a P or Bpulse) is increased (such as by a delta (A)) while the pulse width ofPWM D signal 104 that drives the low side of the bridged output(typically referred to as an N or D pulse) is decreased (such as by thedelta (A)). For negative PWM input signals, a width of the PWM D (or N)signal 104 is increased while the width of the PWM B (or D) signal 102is decreased, resulting two similar but negative differential pulses.Differentially, this is an efficient arrangement since there is nowasted differential energy.

In this example, a differential mode signal 106 includes pulses that arenominally centered at ±T/4, where T is the width of the PWM frame andthe reference time position T=0 represents the center of the frame. Thedifferential mode signal 106 is applied across the load (such as afilter in cascade with a speaker). The carrier frequency of thedifferential mode signal 106 is at twice the PWM frame rate. However,the common mode signal 108 has a peak energy that is nominally centeredat the PWM frame rate. Carrier energy of the common mode signal 108 caninterfere with nearby circuitry or radio receivers.

FIG. 2 illustrates a graph of a resulting differential mode powerspectrum 200 at the output of an associated H-bridge circuit. As shown,the graph 200 illustrates the differential mode component at twice theframe rate in the frequency domain, where the frame rate is 960 kHz.

FIG. 3 illustrates a graph of a resulting common mode power spectrum 300at the output of an associated H-bridge circuit, showing a common modecomponent at the frame rate of 960 kHz. The strong common mode componentcreated at the PWM frame rate, as illustrated by the common mode powerspectrum 300, can interfere with nearby radio receivers. Given thatpractical switching frequencies for audio applications range fromapproximately 200 kHz to 1000 kHz and that the AM band ranges from 520kHz to 1710 kHz, there is a problem with radiated interference of thecommon mode carrier and its harmonics interfering with reception of anAM receiver in close proximity to or within the same system. Therefore,it is desirable to suppress the common mode carrier of a class-BD doublesided symmetrical modulated signal with little or no compromise in thedifferential mode performance. Embodiments described below providesolutions to these and other problems, and offer numerous advantagesover the prior art.

SUMMARY

In a particular embodiment, a circuit device is disclosed that includesa data generator adapted to output a random pulse sequence having aparticular spectral shape. The circuit device further includes a pulseedge control circuit to selectively apply a carrier suppressionoperation to at least one pulse-width modulated (PWM) signal in responseto the random pulse sequence to produce at least one modulated PWMoutput signal. The spectral energy associated with a PWM carrier of themodulated PWM output signal at a carrier frequency and associatedharmonics is changed such that the modulated PWM output signal has aspectral shape defined by the particular spectral shape. In a particularembodiment, the carrier suppression operation includes a phase shiftoperation that is applied to selectively shift the at least one PWMinput signal by plus or minus a quarter of a PWM frame relative to theframe center according to the random pulse sequence. In anotherparticular embodiment, the carrier suppression operation comprises achop operation that is selectively applied to chop or not chop the atleast one PWM input signal with its duty cycle complement PWM signalaccording to the random pulse sequence.

In another particular embodiment, a method is disclosed that includesreceiving at least one pulse-width modulated (PWM) input signal from aPWM source and receiving a random pulse sequence having a particularspectral shape from a data generator. The method further includesapplying a carrier suppression operation to selectively phase shift orto selectively chop the received at least one PWM input signal accordingto values of the random pulse sequence to produce at least one modulatedPWM output signal with a desired spectral shape as defined by the randompulse sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of a particular representative embodiment of aconventional BD-D PWM signal where pulse widths of two pulse waves arevaried, which pulse waves are time-aligned and often centered within apulse width modulated (PWM) frame;

FIG. 2 is a graph of a differential mode (DM) power spectrum of the PWMsignals illustrated in FIG. 1 with a time-varying delta (Δ) and a framerate of 960 kHz;

FIG. 3 is a graph of a common mode (CM) power spectrum of the PWMsignals illustrated in FIG. 1 with a time-varying delta (Δ) and a framerate of 960 kHz;

FIG. 4 is a timing diagram of a particular illustrative embodiment of achop/no chop carrier suppression operation that can be selectivelyapplied to suppress carrier power of a modulated PWM output signal andto spread carrier energy to frequencies other than the carrier frequencyand its associated harmonics within a PWM output spectrum;

FIG. 5 is a graph of a particular illustrative embodiment of a timingdiagram illustrating the basic concept for a quarter-frame phase shiftof a single PWM signal to suppress a carrier at the frame rate;

FIG. 6 is a graph of a particular illustrative embodiment of a spectralshape of a shaped random pulse sequence that can be used to shape apower spectrum of at least one PWM signal to produce at least onemodulated PWM signal having a desired spectral shape;

FIG. 7 is a block diagram of a particular illustrative embodiment of asigma-delta circuit adapted for use as a shaped random pulse sequencegenerator that is programmable to produce a random pulse sequence havinga particular spectral shape;

FIG. 8 is a block diagram of a system including a pulse edge controlcircuit that is responsive to a data generator, such as the sigma-deltacircuit illustrated in FIG. 7, to selectively phase shift or toselectively chop at least one PWM signal according to values associatedwith the random pulse sequence to produce at least one modulated PWMoutput having the particular spectral shape;

FIG. 9 is a graph of a particular illustrative example of a common modepower spectrum associated with a modulated PWM output signal produced byselectively chopping or not chopping a PWM signal and its duty cyclecomplement (within limits of time quantization effects) according tovalues of a random pulse sequence produced by a data generator, such asthe sigma-delta circuit illustrated in FIG. 7; and

FIG. 10 is a flow diagram of a particular illustrative embodiment of amethod of shaping an output power spectrum associated with at least onemodulated PWM output signal.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 4 is a timing diagram 400 of a particular illustrative embodimentof a chop/no chop carrier suppression operation that can be selectivelyapplied to suppress carrier power of a modulated PWM output signal andto spread carrier energy to frequencies other than the carrier frequencyand its associated harmonics within a PWM output spectrum. The timingdiagram 400 includes a high side signal (P) 402 and a low side signal(N) 404, which have a differential mode component represented by signal406 and a common mode component represented by signal 408. Thedifferential mode signal 406 is defined by the following equation:

DM(t)=P(t)−N(t)  (Equation 1).

As shown, in the “not chopped” version, the common mode component(signal 408) has a peak amplitude centered at a center of the frame. Thecommon mode signal 408 is defined by the following equation:

CM(t)=(P(t)+N(t))/2  (Equation 2).

The timing diagram 400 also includes a chopped version of the high andlow side signals (P and N) 402 and 404, represented by the high sidesignal (P′) 412 and the low side signal (N′) 414. In this example, thehigh side signal (P) 402 is inverted and swapped with the low sidesignal (N) 404 and becomes the low side signal (N′) 414, as shown by thefollowing equation:

N′(t)=−P(t)  (Equation 3).

The low side signal (N) 404 is inverted and swapped with the high sidesignal (P) 402 becomes the high side signal (P′) 412, as shown by thefollowing equation:

P′(t)=−N(t)  (Equation 4).

In the chopped version, the differential mode signal 416 remainsunchanged relative to the “not chopped” version, as defined by thefollowing equation:

DM′(t)=P′(t)−N′(t)=−N(t)−−P(t)=DM(t)  (Equation 5).

However, the common mode signal 418 is inverted relative to the commonmode component of the “not chopped” version represented by the signal408, as defined by the following equation:

CM′(t)=(−N(t)−P(t))/2=−CM(t)  (Equation 6).

In this example, when the signal is chopped, the common mode signal 418is inverted and the differential mode signal 416 remains unchanged(relative to the differential mode signal 406). The differential modesignal 406 or 416 determines the audio performance in an audioapplication, and the common mode signal 408 is what is predominantlyresponsible for electromagnetic interference (EMI). In a particularembodiment, by selectively chopping and not chopping a PWM input signaland its duty cycle complement PWM signal, the common mode carrier energyat the carrier frequency is reduced over a sequence of frames, reducingEMI and radio frequency interference. As used herein, the term “dutycycle complement” refers to a signal that, when aggregated with the PWMinput signal, aggregates to an entire width of the PWM frame (withinlimits of time quantization effects). Further, as used herein, the term“chop” or “chop operation” refers to a technique that inverts the PWMinput signal and its duty cycle complement and interchanges them toproduce a modulated PWM output. In a particular example, if the chopoperation is alternately applied every other frame (e.g., a first PWMpulse is not chopped and a second PWM pulse is chopped), a resultingcommon mode carrier energy associated with the PWM P and N signals 402and 404 (and their inverted and swapped (interchanged) versions PWM P′and N′ signals 412 and 414) averages to zero at the carrier frequency.

FIG. 5 is a graph of a particular illustrative embodiment of a timingdiagram 500 illustrating a quarter frame phase shift of a singlepulse-width modulated (PWM) signal to suppress a carrier at the framerate. The timing diagram 500 includes a PWM D signal 502 that iscentered (positioned) within the frame at T/2. Over a two frameinterval, the PWM D signal 502 is shifted. In one example, the PWM Dsignal 502 is shifted left (by −T/4) then right (by +T/4), asillustrated at 504. In another example, the PWM D signal 502 is shiftedright (by +T/4) then left (by −T/4), as illustrated at 506.

In this particular example, the pulse width of the PWM D signal 502 isless than half of the frame width (T/2), so shifting the PWM D signal502 early or late does not introduce any frame edge boundary issues. Inother words, shifting of the PWM D signal 502 does not cause any portionof the pulse to cross the frame boundary (such as the PWM frameboundaries at T=0, T, or 2T, illustrated in FIG. 5). The above examplerepresents a quarter-frame symmetrical pulse shift with no wrap-around.

However, when the pulse width is greater than T/2, then boundaryproblems can arise. For example, if the PWM D signal 502 is wider thanT/2, shifting the PWM D signal 502 by a quarter of the frame width wouldcause a portion of the PWM D signal 502 to extend over the frameboundary (e.g., to cross t=0 or t=T). To avoid having the portion crossthe frame boundary, the PWM D signal 502 can be shifted early (left) orlate (right) by a phase that is less than a quarter of the frame, i.e.,less than +T/4, so that the PWM D signal 502 abuts, but does not cross,the frame boundary. When two signals (a PWM D signal 502 that is widerthan T/2 and a PWM B signal that is narrower than T/2) are shifted, bothsignals may be shifted to abut the frame boundary, such that the PWM Dsignal 502 is shifted by less than a quarter of the frame width and thePWM B signal is shifted by more than a quarter of the frame width. Inthis instance, the sum of the PWM D signal 502 and PWM B signal over twoframes has zero content at the frame repetition rate in the FourierTransform, which cancels the carrier in the common mode signal. Thisparticular example can be referred to as a quarter frame asymmetricalpulse shifting with no wrap-around.

Alternatively, the PWM D signal 502 can be shifted by plus or minus aquarter of the frame, and any portion of the PWM D signal 502 thatcrosses the frame boundary can be wrapped to an opposing frame boundarywithin the same PWM frame. This alternative example can be referred toas a quarter-frame symmetrical pulse shift with wrap-around.

In a particular example, a pulse edge control circuit may be adapted toselectively apply a carrier suppression operation that includesselectively shifting one or more PWM signals by plus or minus a quarterof the frame width using quarter-frame symmetrical pulse shifting withno wrap-around, quarter-frame asymmetrical pulse shifting with nowrap-around, or quarter-frame symmetrical pulse shifting withwrap-around, depending on the implementation.

FIG. 6 is a graph of an illustrative embodiment of a particular spectralshape 600 of a shaped random pulse sequence that can be used to definethe power spectrum of at least one PWM input signal to produce at leastone modulated PWM output signal having a desired spectral shape. Thespectral shape 600 includes two band-stop regions, one with attenuationfrom 0-20 kHz resulting from nulls or notches at DC (0 kHz) and near 20kHz (generally indicated at 602), and a second with attenuation around aselected frequency of interest resulting from nulls or notches at 200kHz+/−10 kHz (generally indicated at 604 and 606). In a particularembodiment, it is desirable to suppress the large tone at the PWM framerate and its harmonics to reduce peak radiated energy. In anotherparticular embodiment, it is desirable to further attenuate spectralenergy within a selected frequency band for improved AM radio reception.Further, in a particular embodiment, it is desirable to have little orno noise within a frequency band from approximately 0-20 kHz to preventaudible noise from coupling into a speaker in an audio application.

FIG. 7 is a block diagram of a particular illustrative embodiment of asigma-delta circuit 700 adapted for use as a shaped random pulsesequence generator that produces a random pulse sequence having aprogrammable spectral shape, such as the particular spectral shape 600illustrated in FIG. 6. In preferred embodiments, the stop-band at 200kHz+/−10 kHz in FIG. 6 can be programmed for specific locations toreduce PWM radiation in desired frequency bands. The sigma-delta circuit700 includes a quantizer 702 to produce a random pulse sequence havingvalues of plus or minus one at an output 704. The sigma-delta circuit700 further includes a feedback loop 705 that has a transfer function(1−G(z)) 706. In this embodiment, the transfer function 706 isprogrammable via a transfer function control input 716 to alter thetransfer function of the feedback loop 705. The sigma-delta circuit 700includes a noise input 708 and a signal input 710 having a zero inputvalue. The signal input 710 is coupled to a first summing node 712 thatproduces a first result that is a difference between a feedback valuereceived from the feedback loop 705 (from the transfer function 706) andthe zero input value. The first result is provided to a summing node724, which subtracts a value at the output 704 from the first result toproduce a feedback result that is provided to the transfer function 706.Additionally, the first result is provided to a second summing node 714,which adds the first result to a noise signal from the noise input 708to produce a second result. The second result is provided to thequantizer 702.

In a particular embodiment, the sigma-delta circuit 700 can beimplemented as digital circuits, analog circuits, firmware, or anycombination thereof. In another particular embodiment, the transferfunction 706 is configurable (programmable) via the transfer functioncontrol input 716 to produce a particular spectral shape, which may ormay not have notches at particular frequencies. The random pulsesequence at the output 704 is consequently shaped by the transferfunction 706. The output 704 may be coupled to a pulse edge controlcircuit that is adapted to selectively apply a carrier suppressionoperation (such as a selective phase shift operation or a selectivechop/no chop operation) according to values of the random pulsesequence.

FIG. 8 is a block diagram of a system 800 including a pulse edge controlcircuit 806 that is adapted to apply a carrier suppression operation toan input PWM signal according to values of a shaped random pulsesequence generator to produce at least one modulated PWM output havingthe particular spectral shape as defined by the data generator. Thesystem 800 includes a pulse-width modulated (PWM) source 802 thatprovides at least one PWM signal 804 to a pulse edge control circuit806. The system 800 also includes the sigma-delta circuit 700illustrated in FIG. 7 that provides a random sequence with a particularspectral shape 704 to the pulse edge control circuit 806. The resultingoutput spectrum of signal 808 is effectively the convolution of theinput PWM spectrum with the spectrum of the random pulse sequence.

In a particular example, the pulse edge control circuit 806 is adaptedto selectively phase shift the at least one PWM signal 804 by plus orminus a quarter of a PWM frame width relative to a center of the PWMframe at integer sub-multiples of a frame repetition rate. In anotherparticular example, the pulse edge control circuit 806 is adapted toselectively chop or not chop the at least one PWM signal 804. In aparticular example, the shift or the chop can be selectively applied bythe pulse edge control circuit 806 based on values of the random pulsesequence with the particular spectral shape 704. The resulting modulatedPWM output signal has a suppressed carrier energy at the carrierfrequency, which energy is spread to other frequencies and the overallspectral shape at 808 is defined by the spectral shape of the datagenerator output, 700.

FIG. 9 is a graph of a particular illustrative example of a powerspectrum 900 of the output PWM signal 808 in FIG. 8. In this case, theprogrammable stop-band was set to be 200 kHz and the PWM frame rate is960 kHz. The common mode power spectrum 900 has been spread as comparedto the common mode power spectrum 300 illustrated in FIG. 3. Further,the common mode power spectrum 900 does not include large common modecomponents that contribute to AM interference (AMI) or electromagneticinterference (EMI). Further, the common mode power spectrum 900 includeslittle noise in the audio frequency band, and notches have been placedat n*960 kHz+/−200 kHz, where n is a non-negative integer, as indicatedat 904, 906, 908, 910, and 912. Further, the graph 900 includes a notchat 0 kHz and at 20 kHz, as indicated at 902. In an alternate example,should the programmable stop-band be centered at 300 kHz, the notcheswould be located at n*960 kHz+/−300 kHz.

FIG. 10 is a flow diagram of a particular illustrative embodiment of amethod of shaping an output power spectrum associated with at least onemodulated PWM output signal. At 1002, at least one pulse-width modulated(PWM) input signal is received from a PWM source. Continuing to 1004, arandom pulse sequence having a particular spectral shape is receivedfrom a data generator. In a particular embodiment, the particularspectral shape includes notches at selected frequencies. Proceeding to1006, a carrier suppression operation is applied to selectively phaseshift or to selectively chop the received at least one PWM input signalaccording to values of the random pulse sequence to produce at least onemodulated PWM output signal with a desired spectral shape as defined bythe random pulse sequence. In a particular embodiment, the at least onemodulated PWM output signal has carrier energy that is spread tofrequencies other than a carrier frequency and its harmonics. In aparticular embodiment, the carrier suppression operation can be appliedat integer sub-multiples of a PWM frame repetition rate or at a ratethat is faster than the frame repetition rate. The method terminates at1008.

In a particular embodiment, the method further includes programming thedata generator to produce the particular spectral shape. In anotherparticular embodiment, the data generator includes a feedback loophaving a programmable transfer function. In still another particularembodiment, the data generator has a nominally white noise input. Thedata generator shapes the white noise source to produce an output pulsesequence having the desired spectral shape often with notches atprogrammable frequency locations.

In conjunction with the systems, circuits, and methods described abovewith respect to FIGS. 4-10, a circuit device is disclosed that isadapted to utilize a random data sequence having a particular spectralshape to control application of a carrier suppression operation. In aparticular example, a pulse edge control circuit selectively phaseshifts a pulse-width modulated (PWM) input signal and its PWM duty cyclecomplement by plus or minus a quarter of a frame width at integersub-multiples of a frame repetition rate based on values of the randomdata sequence. In another particular example, the pulse edge controlcircuit selectively chops (i.e., chops or does not chop) at least onePWM input signal based on values of the random data sequence. In eitherinstance, the resulting modulated PWM output has an altered carrierspectrum that has a spectral shape defined by the particular spectralshape of the random data sequence, including any frequency notches inthe particular spectral shape. The resulting modulated PWM output signalhas reduced carrier energy at a carrier frequency and at harmonics ofthe carrier frequency and exhibits reduced AM interference (AMI) andreduced electromagnetic interference (EMI) with respect to adjacentcircuitry.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A circuit device comprising: a data generator adapted to output arandom pulse sequence having a particular spectral shape; and a pulseedge control circuit to selectively apply a carrier suppressionoperation to at least one pulse-width modulated (PWM) signal in responseto the random pulse sequence to produce at least one modulated PWMoutput signal; wherein spectral energy associated with a PWM carrier ofthe modulated PWM output signal at a carrier frequency and associatedharmonics is changed such that the modulated PWM output signal has aspectral shape defined by the particular spectral shape.
 2. The circuitdevice of claim 1, wherein the random pulse sequence is output at aninteger sub-multiple of a PWM frame repetition rate.
 3. The circuitdevice of claim 1, wherein the particular spectral shape includesnotches at specified frequencies, and wherein the spectral shape of themodulated PWM output signal includes notches at desired frequencies. 4.The circuit device of claim 1, wherein the carrier suppression operationcomprises a phase shift operation that is applied to selectively shiftthe at least one PWM input signal by plus or minus a quarter of a PWMframe relative to the frame center according to the random pulsesequence.
 5. The circuit device of claim 1, wherein the carriersuppression operation comprises a chop operation that is selectivelyapplied to chop or not chop the at least one PWM input signal with itsduty cycle complement PWM signal according to the random pulse sequence.6. The circuit device of claim 5, wherein the chop operation comprisesinverting the at least one PWM signal and its duty cycle complement PWMsignal and interchanging the duty cycle complement PWM signal with theat least one PWM signal to produce the at least one PWM output signal.7. The circuit device of claim 1, wherein the data generator comprises asigma-delta modulator including a zero input signal, a random noisesignal, and a noise transfer function defined by a feedback loop.
 8. Thecircuit device of claim 7, wherein the sigma delta modulator isimplemented with digital circuits.
 9. The circuit device of claim 7,wherein the sigma delta modulator is implemented with analog circuits.10. The circuit device of claim 7, wherein the sigma delta modulator isimplemented with firmware.
 11. The circuit device of claim 7, whereinthe noise transfer function is programmable to create a plurality ofspectral shapes, wherein at least one of the plurality of spectralshapes includes one or more notches at desired frequencies.
 12. A pulseedge control circuit comprising: an input to receive a random sequencehaving a particular spectral shape; and logic to selectively apply acarrier suppression operation to at least one PWM signal in response toreceiving the random sequence to produce at least one modulated PWMoutput signal, the at least one modulated PWM output signal having anenergy spectrum defined by the particular spectral shape.
 13. The pulseedge control circuit of claim 12, wherein energy associated with a PWMcarrier of the modulated PWM output signal is spread in frequency tosuppress energy at carrier frequency and associated harmonics.
 14. Thepulse edge control circuit of claim 12, wherein the particular spectralshape of the random sequence includes notches at selected frequencies,and wherein the spectral shape of the modulated PWM output signalincludes notches at desired frequencies.
 15. The pulse edge controlcircuit of claim 12, wherein the at least one PWM signal includes afirst PWM signal and a second PWM signal, and wherein the carriersuppression operation comprises a phase shift operation that is appliedto selectively shift the first and second PWM signals by plus or minus aquarter of a PWM frame relative to the frame center according to valuesof the random pulse sequence.
 16. The pulse edge control circuit ofclaim 12, wherein the carrier suppression operation comprises a chopoperation that is selectively applied to chop or not chop the at leastone PWM input signal and its duty cycle complement PWM signal accordingto values of the random pulse sequence, wherein the chop operationincludes inverting and interchanging the at least one PWM input signalwith the duty cycle complement PWM signal.
 17. The pulse edge controlcircuit of claim 12, wherein the random sequences comprises a randomsequence of values of either plus or minus one.
 18. A method comprising:receiving at least one pulse-width modulated (PWM) input signal from aPWM source; receiving a random pulse sequence having a particularspectral shape from a data generator; and applying a carrier suppressionoperation to selectively phase shift or to selectively chop the receivedat least one PWM input signal according to values of the random pulsesequence to produce at least one modulated PWM output signal with adesired spectral shape as defined by the random pulse sequence.
 19. Themethod of claim 18, wherein the particular spectral shape includesnotches at selected frequencies.
 20. The method of claim 18, furthercomprising programming the data generator to produce the particularspectral shape.
 21. The method of claim 20, wherein the data generatorincludes a feedback loop including a programmable transfer function, andwherein programming the data generator includes programming theprogrammable transfer function.
 22. The method of claim 18, wherein theat least one modulated PWM output signal has carrier energy that isspread to frequencies other than a carrier frequency and its harmonics.